USGS Professional Paper 1697 - Alaska Resources Library

254 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
steeply dipping major shear zone named the Cadwallader
Break. The deposits have major metals of Au-W-Mo-As and
Au/Ag ratios of 2 to 5. A district-wide mineral zoning varies
from high-temperature Au-As-W-Mo through intermediate
Sb-Ag-Au-As, to low-temperature Sb-Hg vein assemblages
(Woodsworth and others, 1977). The southwest to northeast
zonation is interpreted as forming in the thermal aureole of
Late Cretaceous to early Tertiary plutons of the Coast Plutonic
Complex. The Bralorne deposit is associated with Late
Cretaceous (86-91 Ma) porphyry dikes. The Bralorne-Pioneer
district mines produced 129.96 tonnes Au between 1899 and
1978 from 7.319 million tonnes ore (Dawson and others,
1991). Proven and probable reserves for Bralorne in 1991
were 965,000 tonnes grading 9.3 g/t Au (MINFILE, 2002).
Fish Lake Porphyry Cu-Au (Ag-Mo-Zn) Deposit
The Fish Lake porphyry Cu-Au (Ag-Mo-Zn) deposit
consists of pyrite and chalcopyrite with minor molybdenite,
bornite, sphalerite, and tetrahedrite that occur in stockwork
veins (EMR Canada, 1989; McMillan, 1991; Taseko Mines
Ltd., news release, May 4, 1993; Caira and others, 1995;
MINFILE, 2002). The deposit is hosted mainly in (1) contact
metamorphosed Early Cretaceous(?) andesite flows
and volcaniclastic rocks that occur in an embayment in a
porphyritic quartz diorite stock, (2) an associated, east-westelongated
complex of subparallel quartz-feldspar porphyry
dikes, and (3) disseminations in a Late Cretaceous quartzdiorite
porphyry and in adjacent contact-metamorphosed
Early Cretaceous sedimentary and volcanic rocks (McMillan,
1991; Taseko Mines Ltd., news release, May 4, 1993). The
quartz diorite stock exhibits a U-Pb zircon isotopic age of
about 80 Ma (Schiarizza and Riddell, 1997), and the biotite
hornfels exhibits a K-Ar whole-rock isotopic age of 77.2 Ma
(Wolfhard, 1976). The principal orebody is ovoid shaped with
dimensions of 1,500 m by 800 m and a maximum depth of
880 m. Estimated resource are 1,148 million tonnes grading
0.22 percent Cu and 0.41 g/t Au (Wolfhard, 1976; McMillan,
1991; Caira and others, 1995).
Maggie Porphyry Cu-Mo Deposit
The Maggie (Bonaparte River) porphyry Cu-Mo deposit
consists of chalcopyrite and molybdenite occurring in fine
disseminations in quartz veins and in host rock and in narrow
veinlets in or bordering quartz and calcite veins. The deposit
is hosted by the early Tertiary Maggie quartz monzonite stock,
with a K-Ar isotopic age of 61 Ma, that intrudes metasedimentary
and metavolcanic rocks of Cache Creek terrane (Miller,
1976). The stock occurs several tens of kilometers east of
other plutons in the Fish Lake-Bralorne metallogenic belt.
Chalcopyrite and molybdenite occur in quartz veins and are
disseminated in the stock. High Cu and Mo grades occur in
overlapping potassic and phyllic alteration, and lower Cu and
Mo grades occur in phyllic and argillic alteration. Estimated
resources are 181 million tonnes grading 0.28 percent Cu and
0.029 percent MoS2 (Miller, 1976).
Poison Mountain Porphyry Cu-Mo (Ag-Au) Deposit.
The Poison Mountain Cu-Mo (Ag-Au) porphyry deposit
consists of pyrite, chalcopyrite, molybdenite, and bornite that
occur in veinlets, fracture fillings, and disseminations (Brown,
1995; MINFILE, 2002). The deposit is concentrated at the
contacts between a quartz diorite porphyry stock and dikes, with
isotopic ages of 59 Ma, and contact-metamorphosed Early Cretaceous
graywacke of the Jackass Mountain Group, which is part
of the Methow terrane. The deposit is surrounded by concentric
zones of potassic, phyllic, and propylitic alteration. Concentric
zones of copper sulfide and minor oxide minerals surround a barren
granodiorite core. Estimated reserves, at 0.15 percent Cu cutoff,
are (1) in the oxide zone, 40.2 million tonnes grading 0.228
percent Cu (s), 0.15 percent Cu (ox), 0.127 g/t Au, and 0.007
percent Mo, and (2) in the sulfide zone, 768.3 million tonnes
grading 0.232 Cu, 0.122 g/t Au, and 0.007 percent Mo (Brown,
1995; Seraphim and Rainboth, 1976; McMillan, 1991). The
stock and associated dikes exhibit K-Ar isotopic ages that range
from a hornblende age of 61.4 for biotite-altered quartze diorite
to a biotite-hornblende age of 55.5 Ma for contact-metamorphosed
sedimentary rock in the outer part of the deposit (Brown,
1995). The deposit occurs 75 km southeast of Fish Lake.
Origin of and Tectonic Controls for Fish Lake-Bralorne
Metallogenic Belt
The Fish Lake-Bralorne metallogenic belt is defined by
the distribution of a suite of small, Late Cretaceous to Eocene,
quartz monzonite and quartz diorite stocks that are part of the
Coast Plutonic Complex but occur east of the main part of the
batholith. The plutons are essentially coeval with plutons marking
the eastern edge of the Coast suite and probably represent
the eastern limit of Late Cretaceous-Eocene magmatism in the
southern part of the continental-margin arc. The Coast Plutonic
Complex is part of the Coast-North Cascade plutonic belt.
Three major pulses of mineralization are interpreted for
the Fish Lake-Bralorne belt (Schiarizza and others, 1997)—(1)
The older, Late Cretacous deposits in the belt are interpreted
as forming along early Late Cretaceous reverse to sinistral
faults that are interpreted as forming during the last part of
contractual deformation that was associated with subduction,
(2) The younger, latest Cretaceous to Paleocene deposits in
the belt occur along dextral-slip faults, such as the Castle Pass
fault and may in part be controlled by an extensional bend in
the fault system (Schiarizza and others, 1997), and (3) the still
younger, porphyry occurrences and associated polymetallic
vein deposits are associated with Middle Eocene granodiorite
plutons that occur along dextral fault systems.
Tyaughton-Yalakom Metallogenic Belt of W-Sb
Polymetallic Vein and Hg-Sb Vein Deposits (Belt
TY), Southern British Columbia
The Tyaughton-Yalakom metallogenic belt of W-Sb polymetallic
vein and Hg-Sb vein deposits occurs in southern Brit-